Last Day Factors that affect reaction rates Nature of reactants Ionic bonds = fast Covalent bonds = slow (unless combustion rxn) Liquids, solution, or gas react quicker than solids Increased surface area = increased rate of reaction Stirring increases frequency of collisions = increased rate Temperature Concentration + Pressure Catalyst

Reversible Reactions & Equilibrium Unit 3

Reversible Reactions One misconception about chemical reactions is that they can only happen in one direction. That is, in previous science courses one way we defined a chemical change is that they cannot be reversed. For example, frying an egg.

Reversible Reactions In this unit we will learn that reactions can be reversed under the right conditions. For example, 2NO2  N2O4 and N2O4  2NO2. In a situation like this, we can combine the two ideas by writing the equation using a double arrow (↔): 2NO2 ↔ N2O4 Forward vs. reverse reaction Note: forward reaction will always be read from left to right

Where have we seen equilibrium already?

Equilibrium If we add salt to a beaker of water, we will have the reversible reaction: NaCl(s) ↔ Na+(aq) + Cl -(aq) However, if we keep adding salt, we will reach a point where no more salt is able to dissolve and the excess sits at the bottom of the beaker. One question we must ask is whether or not the dissolving process has stopped. When a solution becomes saturated, it has reached the point of equilibrium. This means that the forward reaction... NaCl(s)  NaCl(aq) and the reverse reaction... NaCl(aq) NaCl(s) happen at the same rate.

So what happens... When you first put reactants together the forward reaction starts. Since there are no products there is no reverse reaction. As the forward reaction proceeds the reactants are used up so the forward reaction slows. The products build up, and the reverse reaction speeds up.

Eventually you reach a point where the reverse reaction is going as fast as the forward reaction. The rate of the forward reaction is equal to the rate of the reverse reaction. This is dynamic equilibrium The concentration of products and reactants stays the same, but the reactions are still running.

Dynamic Equilibrium This is why we think the dissolving process has stopped: we don’t see any observable change in the amount of salt at the bottom of the solution. Only microscopic changes occur at equilibrium. This may also be called dynamic equilibrium, but we will just refer to it as equilibrium. Note that we are talking about equal rates, not equal reactants and products.

Equilibrium In order for equilibrium to be reached with a reversible reaction the system must be closed. That is, the reactants and products cannot be released into the environment or surroundings Reactants and products must be contained An analogy for equilibrium: subbing players into a sports game. The players represent the reactants and products As one player is subbed off the field, another one takes their place The total number of players remains constant, and the game itself represents the closed system

Steady State Systems Imagine if the product was removed as soon as it was formed, could it be reversed into the reactants? The answer is NO If you continuously add reactants to a system at the same rate you remove the products you create a steady state system. A factory with an assembly line is an example of a steady state system. That is, raw materials are constantly being added to make products which are constantly being removed. However, a steady state system is not equilibrium because the reverse reaction does not happen.